There are a number of high current loads positioned throughout a vehicle which are controlled by input switches located remote from the loads As used herein, the term "high current load" is intended to refer to both the circuitry that controls the delivery of power to the loads (e.g., relays, integrated circuit drivers, and driver circuitry comprising discrete semiconductor devices which control power delivery to the loads in response to low power digital inputs) and the associated output device to which power is delivered. Examples of vehicle output devices include bidirectional DC inductive motors for operating power seats, power windows and windshield wipers, solenoids for operating door and trunk locks, large lamp loads, etc.
Conventionally, vehicle output devices have been operated using direct high current control which involves switching a source voltage at a high current switch so that the entire current drawn by the device passes through the switch Where direct high current control is not practical due to the distance between the switch and the device, power relays have been placed near the device and a high current switch is used to switch the relay on and off, thereby switching the device on and off.
Direct high current control provides a relatively simple and reliable method of operating vehicle output devices; however, high current switches are bulky and heavy, and contain large volumes of expensive metals. High current switches and relays also typically require a relatively large "wetting" current, i.e., a minimum current supplied to the switch or relay contacts when closed to prevent oxidation of the contacts after repeated operation of the switch or relay. Additionally, many complex memory functions and control routines used to control output devices in most automobiles, and particularly in luxury automobiles, are not available using direct high current control.
In response to the limitations associated with direct high current control, the aforementioned integrated circuit drivers were developed to power and control vehicle output devices. Such integrated circuit drivers comprise both digital and analog circuity for providing power outputs for powering vehicle output devices in response to low power digital inputs. Additionally, certain relays are now available for directly switching power to vehicle output devices in response to low power digital inputs.
Typically a CPU is used to provide digital inputs to integrated circuit drivers or relays. As is known in the art, the term "CPU" is an abbreviation for "central processing unit". As used herein the term "CPU" is intended to refer to signal processors and other suitable signal processing means for providing desired control functions. The CPU provides desirable memory functions and control routines for controlling high current loads. Low current switches used to provide input signals to the CPU are smaller and less expensive than high current switches. Additionally, low current switches require less "wetting" current and have less open circuit leakage than high current switches, and therefore place a lower power demand on the vehicle electrical system.
A problem associated with low current switching is that a failure of the intermediate circuitry, e.g., the CPU between the switches and a driver or relay, completely disables function of the high current loads. These failures are of particular concern when safety related devices are disabled, e.g., windshield wipers, lamps, rear view mirror positioning, seat positioning, etc. Such failures may render the vehicle inoperable until function of the safety related items is restored.